When manufacturing magnetic heads for magnetic storage applications, a critical step is a milling (lapping) process in which material from one side of the head is trimmed to form an air bearing surface (ABS). Typically, a plurality of heads is arranged side by side in row that has been sliced from a substrate and mounted on a lapping plate in front of a lapping tool. Once the lapping process is complete, the row is diced to form individual heads. Each head is formed on a slider which in the final device is attached to a servo control unit that guides the head over a spinning recording medium during a read or write operation.
During the lapping process, a plurality of electrical lapping guides (ELGs) which were placed along the ABS in the preceding head fabrication steps and which are attached to a controller that guides the lapping tool are used to determine when the lapping process is complete. Typically, the read head is lapped along an ABS plane to provide an acceptable sensor stripe height (SH) which is the distance from the ABS to the back of the sensor. In a merged read/write head structure, nearby layers in the  write head are simultaneously lapped to determine critical dimensions such as the throat height (TH) and neck height in the second pole piece of the write head. The neck height (NH) is the distance from the ABS to the back side of the neck region where the second pole piece begins to widen into the yoke region. The throat height is the distance from the ABS toward the back side of the yoke region where the second pole piece begins to separate from the first pole piece. Each of the SH, NH, and TH distances has a tight tolerance in order to optimize the magnetic head performance.
A typical ABS lapping process is designed to accurately control the read element stripe height alone and the control on some critical dimensions of the write head is therefore looser. The read head stripe height as well as the wafer level alignment usually dictates the write head neck height and throat height which cannot be independently controlled. Furthermore, any in-process misalignment between the wafer plane and the ABS lapping plane also results in added variations of some critical write head dimensions.
A conventional perpendicular magnetic recording (PMR) device with a merged read/write head 1 is depicted in FIG. 1. The read head is formed first on a substrate 2 that has a top surface 2a. There is a first shield layer 3 formed on the substrate 2 and first and second gap layers 4a, 4b consecutively formed on the first shield layer. Between the first and second gap layers 4a, 4b is a sensor 5 with a stripe height SH. A second shield layer 8 forms the top of the read head. The read head is separated from the write head by a separation layer 7 having a thickness d which is known as the read-write separation distance. 
The bottom layer in the write head is a bottom yoke 8 which is recessed from the ABS plane A–A′ by a distance c which is typically about 1 micron. Adjacent to the bottom yoke 8 on the separation layer 7 is formed a non-magnetic write gap layer 9 that extends from the ABS toward the back side of the write head. A main pole piece 10 on the write gap layer 9 has a width a in a pole tip region (FIG. 2) at the ABS and begins to diverge and form a wider width at a neck height distance NH from the ABS plane A–A′. There is successively formed a first insulation layer 11 and a second insulation layer 12 on the main pole piece 10. There is a plurality of coils 13 located within the second insulation layer 12 which are wrapped around a back gap region (not shown) where the main pole piece 10 joins a top yoke section 15. An overcoat dielectric layer 14 such as alumina typically covers the coils 13. Also shown are a first write shield layer 16 which is a magnetic layer that extends a distance TH from the ABS plane on the main pole piece 10 and a second write shield layer 17 between the first write shield layer and the top yoke 15.
Ideally, the lapping process results in an ABS plane A–A′ which is perpendicular to the surface 2a of the substrate 2. However, due to lapping process variations, an ABS plane B–B′ may be formed in which the throat height TH and neck height (not shown) in the main pole piece 10 will be shorter than the design value by a distance equal to d ×tan θ where d is the read-write separation distance and θ is the misalignment angle. When d is large enough to place NH or TH below a minimum specified value, then the head may be scrapped since rework is not possible. Therefore, a means for controlling the lapping process is necessary that can independently control TH, NH, and SH. 
Referring to FIG. 2, a cross-sectional view of the merged read/write head 1 is pictured from the ABS plane A–A′. The direction that the head moves over a recording medium is shown by the arrow z. The width a of the main pole piece 10 in a pole tip region at the ABS plane is another critical dimension because it is the track width. A higher recording density is achieved with a narrower track width but a more controlled lapping process is necessary to satisfy tight tolerances for SH, TH, and NH.
A dual element lapping guide system is disclosed in U.S. Pat. No. 6,027,397 and includes resistive elements superimposed on electrical switch elements in kerf areas. The resistive elements are aligned with the MR transducers and the electrical switch elements are aligned with the inductive magnetic transducers.
In U.S. Patent Pub. 2003/0200041, element like ELGs (ELEs) and ELGs are placed in alternating kerfs to improve stripe height calibration. Stripe height data is collected using ELGs and resistive data values are simultaneously collected using the ELEs.
In U.S. Pat. No. 6,609,948, an ELG is formed in the sensor material layer. Various films are employed for the ELG to minimize magnetoresistance and optimize the resistance of the ELG.
U.S. Pat. No. 6,193,584 describes an ELG in which a first resistive element is separated from a second resistive element by a common lead. The initial heights of the two resistive elements are different and are at least 15 microns larger than the target stripe height. This design provides different resistances during the lapping process. 